Academic dissertation to be presented with the assent of the Doctoral Training Committee of Technology and Natural Sciences of the University of Oulu for public defence in the Arina auditorium (TA105), Linnanmaa, on 23 October 2015, at 12 noon

Abstract

Nanocellulose is envisioned as one of the key product innovations of future biorefineries, since it can potentially function in numerous high-end applications and replace many current petroleum-based products due to its superior properties, abundance and renewable nature. The main difficulty hindering the industrial upscaling of nanocellulose is the lack of feasible techniques for processing cellulose fibres on a nanoscale. At the same time, ongoing research efforts have concentrated on charting the suitability of nanocellulose for various novel applications. The chemical functionalization of cellulose is currently regarded as a significant step for both enhancing nanocellulose fabrication and increasing its value as a product by virtue of its adjustable surface properties.

This thesis reports on the surface functionalization of cellulosic fibres by means of two new chemical pre-treatments based on periodate oxidation and sequential chlorite oxidation or reductive amination for use in the fabrication of nanocelluloses. The properties of the resulting nanocelluloses were characterized and their applicability to novel film structures was investigated. Both nanoporous thin films for composite membranes and self-standing barrier films were manufactured and studied for their suitability in water purification and packaging applications, respectively.

The oxidation of cellulose to 2,3-dicarboxylic acid cellulose (DCC) significantly enhanced the nanofibril production as only 1-4 passes through the homogenizer were required for disintegration of the fibres down to nano-scale. The fabricated DCC-nanofibrils had both high optical transmittance and viscosity comparable to that of TEMPO-oxidized cellulose nanofibrils. DCC-nanofibrils with a carboxyl content of 1.75 mmol/g showed a potential for functioning as a nanoporous thin-film membrane layer in ultrafiltration tests. The second pre-treatment introduced an acid-free fabrication of amphiphilic cellulose nanocrystals (CNCs) with uniform width and length into nanocellulose production for the first time. Reaction conditions of periodate oxidation were presumed to be one of the key factors to impact the formation of either CNCs or cellulose nanofibrils. The butylamino-functionalized CNCs were used to fabricate barrier films that showed good mechanical strength and high resistance to permeation by oxygen even at elevated relative humidity.